EliasFano: setBits simplify

[AskAlexSharov]

Changes made:

1. db/recsplit/eliasfano32/elias_fano.go — Replaced setBits with branchless version (removed width param,
   mask computation, and branch), updated 3 callers
2. db/recsplit/eliasfano16/elias_fano.go — Same transformation, updated 3 callers
3. db/recsplit/eliasfano32/elias_fano_seek_bench_test.go — Added BenchmarkAddOffset with stride1/123/1000
   cases

  ┌───────────────┬─────────┬─────────┬─────────┐                                                                                                         
  │    Stride     │ Before  │  After  │ Speedup │                                                                                                         
  ├───────────────┼─────────┼─────────┼─────────┤                                                                                                         
  │ stride1_l0    │ 3.22 ms │ 2.60 ms │ -19%    │                                                                                                         
  ├───────────────┼─────────┼─────────┼─────────┤                                                                                                         
  │ stride123_l6  │ 3.42 ms │ 2.65 ms │ -23%    │                                                                                                         
  ├───────────────┼─────────┼─────────┼─────────┤                                                                                                         
  │ stride1000_l9 │ 3.51 ms │ 2.71 ms │ -23%    │
  └───────────────┴─────────┴─────────┴─────────┘

[Giulio2002]

LGTM — clean simplification of setBits: removes unused width param, uses |= with unconditional second write (safe due to +1 padding). Nice BCE hint addition.